Provided by: freebsd-manpages_12.2-2_all 
NAME
ieee80211_radiotap — 802.11 device packet capture support
SYNOPSIS
#include <net80211/ieee80211_var.h>
void
ieee80211_radiotap_attach(struct ieee80211com *, struct ieee80211_radiotap_header *th, int tlen,
uint32_t tx_radiotap, struct ieee80211_radiotap_header *rh, int rlen, uint32_t rx_radiotap);
int
ieee80211_radiotap_active_vap(struct ieee80211vap *);
int
ieee80211_radiotap_active(struct ieee80211com *);
void
ieee80211_radiotap_tx(struct ieee80211vap *, struct mbuf *);
DESCRIPTION
The net80211 layer used by 802.11 drivers includes support for a device-independent packet capture format
called radiotap that is understood by tools such as tcpdump(1). This facility is designed for capturing
802.11 traffic, including information that is not part of the normal 802.11 frame structure.
Radiotap was designed to balance the desire for a hardware-independent, extensible capture format against
the need to conserve CPU and memory bandwidth on embedded systems. These considerations led to a format
consisting of a standard preamble followed by an extensible bitmap indicating the presence of optional
capture fields. A net80211 device driver supporting radiotap defines two packed structures that it
shares with net80211. These structures embed an instance of a ieee80211_radiotap_header structure at the
beginning, with subsequent fields in the appropriate order, and macros to set the bits of the it_present
bitmap to indicate which fields exist and are filled in by the driver. This information is then supplied
through the ieee80211_radiotap_attach() call after a successful ieee80211_ifattach() request.
With radiotap setup, drivers just need to fill in per-packet capture state for frames sent/received and
dispatch capture state in the transmit path (since control is not returned to the net80211 layer before
the packet is handed to the device). To minimize overhead this work should be done only when one or more
processes are actively capturing data; this is checked with one of ieee80211_radiotap_active_vap() and
ieee80211_radiotap_active(). In the transmit path capture work looks like this:
if (ieee80211_radiotap_active_vap(vap)) {
... /* record transmit state */
ieee80211_radiotap_tx(vap, m); /* capture transmit event */
}
While in the receive path capture is handled in net80211 but state must be captured before dispatching a
frame:
if (ieee80211_radiotap_active(ic)) {
... /* record receive state */
}
...
ieee80211_input(...); /* packet capture handled in net80211 */
The following fields are defined for radiotap, in the order in which they should appear in the buffer
supplied to net80211.
IEEE80211_RADIOTAP_TSFT
This field contains the unsigned 64-bit value, in microseconds, of the MAC's 802.11 Time
Synchronization Function (TSF). In theory, for each received frame, this value is recorded when
the first bit of the MPDU arrived at the MAC. In practice, hardware snapshots the TSF otherwise
and one cannot assume this data is accurate without driver adjustment.
IEEE80211_RADIOTAP_FLAGS
This field contains a single unsigned 8-bit value, containing one or more of these bit flags:
IEEE80211_RADIOTAP_F_CFP
Frame was sent/received during the Contention Free Period (CFP).
IEEE80211_RADIOTAP_F_SHORTPRE
Frame was sent/received with short preamble.
IEEE80211_RADIOTAP_F_WEP
Frame was encrypted.
IEEE80211_RADIOTAP_F_FRAG
Frame was an 802.11 fragment.
IEEE80211_RADIOTAP_F_FCS
Frame contents includes the FCS.
IEEE80211_RADIOTAP_F_DATAPAD
Frame contents potentially has padding between the 802.11 header and the data payload to
align the payload to a 32-bit boundary.
IEEE80211_RADIOTAP_F_BADFCS
Frame was received with an invalid FCS.
IEEE80211_RADIOTAP_F_SHORTGI
Frame was sent/received with Short Guard Interval.
IEEE80211_RADIOTAP_RATE
This field contains a single unsigned 8-bit value that is the data rate. Legacy rates are in
units of 500Kbps. MCS rates (used on 802.11n/HT channels) have the high bit set and the MCS in
the low 7 bits.
IEEE80211_RADIOTAP_CHANNEL
This field contains two unsigned 16-bit values. The first value is the center frequency for the
channel the frame was sent/received on. The second value is a bitmap containing flags that
specify channel properties.
This field is deprecated in favor of IEEE80211_RADIOTAP_XCHANNEL but may be used to save space in
the capture file for legacy devices.
IEEE80211_RADIOTAP_DBM_ANTSIGNAL
This field contains a single signed 8-bit value that indicates the RF signal power at the
antenna, in decibels difference from 1mW.
IEEE80211_RADIOTAP_DBM_ANTNOISE
This field contains a single signed 8-bit value that indicates the RF noise power at the antenna,
in decibels difference from 1mW.
IEEE80211_RADIOTAP_DBM_TX_POWER
Transmit power expressed as decibels from a 1mW reference. This field is a single signed 8-bit
value. This is the absolute power level measured at the antenna port.
IEEE80211_RADIOTAP_ANTENNA
This field contains a single unsigned 8-bit value that specifies which antenna was used to
transmit or receive the frame. Antenna numbering is device-specific but typically the primary
antenna has the lowest number. On transmit a value of zero may be seen which typically means
antenna selection is left to the device.
IEEE80211_RADIOTAP_DB_ANTSIGNAL
This field contains a single unsigned 8-bit value that indicates the RF signal power at the
antenna, in decibels difference from an arbitrary, fixed reference.
IEEE80211_RADIOTAP_DB_ANTNOISE
This field contains a single unsigned 8-bit value that indicates the RF noise power at the
antenna, in decibels difference from an arbitrary, fixed reference.
IEEE80211_RADIOTAP_XCHANNEL
This field contains four values: a 32-bit unsigned bitmap of flags that describe the channel
attributes, a 16-bit unsigned frequency in MHz (typically the channel center), an 8-bit unsigned
IEEE channel number, and a signed 8-bit value that holds the maximum regulatory transmit power
cap in .5 dBm (8 bytes total). Channel flags are defined in: <net80211/_ieee80211.h> (only a
subset are found in <net80211/ieee80211_radiotap.h> ). This property supersedes
IEEE80211_RADIOTAP_CHANNEL and is the only way to completely express all channel attributes and
the mapping between channel frequency and IEEE channel number.
EXAMPLES
Radiotap receive definitions for the Intersil Prism driver:
#define WI_RX_RADIOTAP_PRESENT \
((1 << IEEE80211_RADIOTAP_TSFT) \
(1 << IEEE80211_RADIOTAP_FLAGS) | \
(1 << IEEE80211_RADIOTAP_RATE) | \
(1 << IEEE80211_RADIOTAP_CHANNEL) | \
(1 << IEEE80211_RADIOTAP_DB_ANTSIGNAL) | \
(1 << IEEE80211_RADIOTAP_DB_ANTNOISE))
struct wi_rx_radiotap_header {
struct ieee80211_radiotap_header wr_ihdr;
uint64_t wr_tsf;
uint8_t wr_flags;
uint8_t wr_rate;
uint16_t wr_chan_freq;
uint16_t wr_chan_flags;
uint8_t wr_antsignal;
uint8_t wr_antnoise;
} __packed __aligned(8);
and transmit definitions for the Atheros driver:
#define ATH_TX_RADIOTAP_PRESENT ( \
(1 << IEEE80211_RADIOTAP_FLAGS) | \
(1 << IEEE80211_RADIOTAP_RATE) | \
(1 << IEEE80211_RADIOTAP_DBM_TX_POWER) | \
(1 << IEEE80211_RADIOTAP_ANTENNA) | \
(1 << IEEE80211_RADIOTAP_XCHANNEL) | \
0)
struct ath_tx_radiotap_header {
struct ieee80211_radiotap_header wt_ihdr;
uint8_t wt_flags;
uint8_t wt_rate;
uint8_t wt_txpower;
uint8_t wt_antenna;
uint32_t wt_chan_flags;
uint16_t wt_chan_freq;
uint8_t wt_chan_ieee;
int8_t wt_chan_maxpow;
} __packed;
SEE ALSO
tcpdump(1), bpf(4), ieee80211(9)
HISTORY
The ieee80211_radiotap definitions first appeared in NetBSD 1.5.
AUTHORS
The original version of this manual page was written by Bruce M. Simpson <bms@FreeBSD.org> and Darron
Broad <darron@kewl.org>.
Debian March 11, 2019 IEEE80211_RADIOTAP(9)